The invention relates to an impact additive of the core/shell type as well as to a composition containing a thermoplastic polymer, in particular a vinyl chloride homopolymer or a copolymer mostly containing vinyl chloride, an impact additive of the core/shell type and optionally other additives.
Some synthetic resins, in particular resins based on poly (vinyl chloride) or on a copolymer mostly containing vinyl chloride, are widely used in the building industry, in particular due to their low cost and to their good physical and/or chemical properties.
Nevertheless, they exhibit low impact strengths at ambient temperature or at low temperature or again also after ageing.
It has been proposed to overcome these defects by incorporating, in these thermoplastic resins, products known as impact additives which are generally polymers exhibiting a degree of elastomeric properties.
A description is given in U.S. Pat. No. 3,678,133 of an impact additive of the core/shell type composed of an elastomeric core and of a more rigid thermoplastic shell.
The elastomeric core is obtained by polymerization of a mixture of monomers comprising at least 50% by weight of an alkyl acrylate, the alkyl group of which has from 2 to 8 carbon atoms, and a minor proportion of a crosslinking agent. The preferred alkyl acrylate is n-butyl acrylate.
It is also mentioned that alkyl acrylates having longer chains exhibit the disadvantage of polymerizing with greater difficulty, 2-ethylhexyl acrylate being given as an example.
The rigid thermoplastic shell is obtained by polymerization of a mixture of monomers comprising 40% to 100% by weight of alkyl methacrylate in which the alkyl group contains 1 to 4 carbon atoms.
The impact additive in this patent is produced in such a way that the polymerization of the rigid thermoplastic shell takes place at the surface of the elastomeric phase, preferably as a separate layer which more or less completely covers the elastomeric core.
Although the impact additives thus obtained significantly improve the impact strength at ambient temperature of the resins containing them, there is a loss in the mechanical properties, in particular a loss in the impact strength, at low temperature, of the said resins.
An impact additive of the core/shell type has now been found which is composed of a core based on alkyl acrylate or on a polyorganosiloxane rubber and a shell based on poly(alkyl methacrylate), or on a styrene-acrylonitrile copolymer, characterized in that the said impact additive comprises from:
(a) 70% to 90% by weight, and preferably 75% to 85%, of an elastomeric crosslinked core which is composed:
1) of 20% to 100% by weight, and preferably of 20% to 90%, of a nucleus composed of a copolymer (I) of n-alkyl acrylate, the alkyl group of which has a carbon number ranging from 5 to 12, and preferably ranging from 5 to 8, or of a mixture of alkyl acrylates, the linear or branched alkyl group of which has a carbon number ranging from 2 to 12, and preferably ranging from 4 to 8, or of a polyorganosiloxane rubber, of a polyfunctional crosslinking agent possessing unsaturated groups in its molecule, at least one of which is of CH2xe2x95x90C less than  vinyl type, and optionally of a polyfunctional grafting agent possessing unsaturated groups in its molecule, at least one of which is of CH2xe2x95x90CHxe2x80x94CH2xe2x80x94allyl type, the said nucleus containing a molar amount of crosslinking agent and optionally of grating agent ranging from 0.05% to 5% and preferably an amount of between 0.5% and 1.5%;
2) of 80% to 0% by weight, and preferably of 80% to 10%, of a covering composed of a copolymer (II) of n-alkyl acrylate, the alkyl group of which has a carbon number ranging from 4 to 12, and preferably ranging from 4 to 8, or of a mixture of alkyl acrylates as defined above in 1) and of a polyfunctional grafting agent possessing unsaturated groups in its molecule, at least one of which is of CH2xe2x95x90CHxe2x80x94CH2xe2x80x94allyl type, the said covering containing a molar amount of grafting agent ranging from 0.05% to 2.5% and preferably an amount of between 0.5% and 1.5%;
b) 30% to 10% by weight, and preferably 25% to 15%, of a shell grafted onto the said core composed of a polymer of an alkyl methacrylate, the alkyl group of which has a carbon number ranging from 1 to 4, or alternatively of a statistical copolymer of an alkyl methacrylate, the alkyl group of which has a carbon number ranging from 1 to 4, and of an alkyl acrylate, the alkyl group of which has a carbon number ranging from 1 to 8, containing a molar amount of alkyl acrylate ranging from 5% to 40%, and preferably of between 10% and 20, or alternatively composed of a styrene-acrylonitrile copolymer having a preferred styrene:acrylonitrile molar ratio between 1:1 and 4:1, and particularly between 7:3 and 3:1, respectively.
Mention will be made, as illustration of n-alkyl acrylates which can be used according to the present invention to form the copolymer (I), of n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate and very particularly n-octyl acrylate.
Mention will be made, as illustration of n-alkyl acrylates which can be used according to the present invention to form the copolymer (II), of n-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate and very particularly n-octyl acrylate.
The n-alkyl acrylate which may be used to form the copolymers (I) and/or (II) can be identical or different.
Mention will be made, as illustration of linear or branched alkyl acrylates which can be used according to the present invention for the formation of the mixtures of alkyl acrylates constituting the copolymers (I) and /or (II), of ethyl acrylate, n-propyl acrylate, n-butyl acrylate, amyl acrylate, 2-methylbutyl acrylate, 2-ethylhexyl acrylate, n-hexyl acrylate, n-octyl acrylate, n-decyl acrylate, n-dodecyl acrylate or 3,5,5-trimethylhexyl acrylate.
In the case where a mixture of alkyl acrylates is used to produce the copolymers (I) and/or (II), use will be made of an amount by weight of n-alkyl acrylate at least equal to 10% by weight of the mixture of alkyl acrylates and preferably an amount of between 20% and 80%.
As above, use may be made, to form the copolymers (I) and/or (II), of a mixture of identical or different alkyl acrylates.
According to the present invention, it is preferable to use n-alkyl acrylates and very particularly n-octyl acrylate to form the copolymers (I) and (II).
If a mixture of alkyl acrylates is used to form the copolymers (I) and/or (II), use will preferably be made of 20% to 80% by weight of n-octyl acrylate and preferably of 80% to 20% by weight of n-butyl acrylate.
Mention will be made, as illustration of alkyl methacrylates which can be used to form the shell grafted onto the crosslinked elastomeric core according to the present invention, of ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate and very particularly methyl methacrylate.
According to the present invention, the crosslinking agent used to form the copolymer (I) can in particular be chosen from derivatives possessing at least two double bonds of the vinyl type or alternatively possessing one or a number of double bonds of the vinyl type and at least one double bond of the allyl type. Use will preferably be made of compounds possessing, in their molecules, a majority of double bonds of the vinyl type.
Mention will be made, as illustration of such crosslinking agents, of divinylbenzenes, polyalcohol (meth)acrylates, such as trimethylolpropane, triacrylate or trimethacrylate, allyl acrylate or methacrylate, alkylene glycol diacrylates or dimethacrylates having 2 to 10 carbon atoms in the alkylene chain and in particular ethylene glycol diacrylate or dimethacrylate, 1,4-butanediol diacrylate or dimethacrylate or 1,6-hexanediol diacrylate or dimethacrylate or polyoxyalkylene glycol diacrylate or dimethacrylate of formula 
in which X represents a hydrogen atom or the methyl radical, n is an integer ranging from 2 to 4 and p is an integer ranging from 2 to 20 and in particular polyoxyethylene glycol diacrylate or dimethacrylate in which the polyoxyethylene radical has a molecular mass of approximately 400 (abovementioned formula with n=2 and p=9).
According to the present invention, the grafting agent used to form the copolymer (II) can be in particular chosen from derivatives possessing at least two double bonds of the allyl type or alternatively possessing one or a number of double bonds of the allyl type and at least one double bond of the vinyl type.
Use will preferably be made of compounds possessing, in their molecules, a majority of double bonds of the allyl type.
Mention will be made, as illustration, of such grafting agents, of diallyl maleate, diallyl itaconate, allyl methacrylate or acrylate, triallyl cyanurate, triallyl isocyanurate, diallyl terephthalate or triallyl trimesate.
According to an alternative form in accordance with the invention, the nucleus of the crosslinked elastomeric core can be composed entirely of a polyorganosiloxane rubber obtained by emulsion polymerization of an organosiloxane in the presence of a crosslinking agent and, optionally, of a grafting agent.
Mention may be made, as illustration of organosiloxanes, of cyclic siloxanes composed of rings having a number of Sixe2x80x94C ring members ranging from 3 to 6, such as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, dodecamethylcyclotetrasiloxane or octaphenylcyclotetrasiloxane.
Mention may be made, as crosslinking agent which can be sued, of a crosslinking agent of the tri- or tetrafunctional silane type, such as, for example, trimethoxysilane or tetraethoxysilane.
Use will preferably be made, as grating agent, of a methacryloyloxysiloxane of formula: 
in which R1 represents a methyl, ethyl, propyl or phenyl group, R2 represents a hydrogen atom or a methyl group, n has a value 0, 1 or 2 and p is a number ranging from 1 to 6.
Mention may be made, as illustration of methacryloyloxysiloxane, of:
xcex2-methacryloyloxyethyldimethoxymethylsilane,
xcex3-methacryloyloxypropylmethoxydimethylsilane,
xcex3-methacryloyloxypropyldimethoxysilane,
xcex3-methacryloyloxypropyltrimethoxysilane,
xcex3-methacryloyloxypropylethoxydiethylsilane,
xcex3-methacryloyloxypropyldiethoxymethylsilane, and
xcex4-methacryloyloxybutyldiethoxymethylsilane.
The polyorganosiloxane rubber can be produced by a process described, for example, in European Patent EP 0,326,038. Use will very particularly be made of the procedure described in the example of reference 1 of the said patent, which makes it possible to obtain a polyoctamethylcyclotetrasiloxane rubber latex.
According to this alternative form, the crosslinked elastomeric core can contain no more than 40% by weight of a nucleus composed of a polyorganosiloxane rubber as described above.
The invention also relates to a composition comprising a thermoplastic polymer and the impact additive as defined above.
The thermoplastic polymer can be composed of one or a number of polymers of the polycondensates type, in particular polyamides, polyetheresteramides (PEBAX), polyesters, such as polybutylene terephthalate, polycarbonates or alloys of the abovementioned polymers, such as alloys of polycarbonates and of the polyesters, such as XENOY. The thermoplastic polymer can also be composed of one or a number of polymers chosen from the group formed by poly(alkyl methacrylate)s and in particular poly(methyl methacrylate) or by a vinyl chloride homopolymers, which can optionally be superchlorinated, and copolymers which result from the copolymerization of vinyl chloride with one or a number of ethylenically unsaturated comonomers and which contain at least 80% by weight of polymerized vinyl chloride. Examples of monomers which are suitable for the preparation of such copolymers are in particular vinylidene halides, such as vinylidene chloride or fluoride, vinyl carboxylates, such as vinyl acetate, vinyl propionate or vinyl butyrate, acrylic and methacrylic acids and the nitriles, amides and alkyl esters which derive therefrom, in particular acrylonitrile, acrylamide, methacrylamide; methyl methacrylate, methyl acrylate, butyl acrylate, ethyl acrylate or 2-ethylhexyl acrylate, vinylaromatic derivatives, such as styrene or vinylnaphthalene, or olefins, such as bicyclo[2.2.1]hept-2-ene, bicyclo[2.2.1]hepta-2,5-diene, ethylene, propene or 1-butene.
The thermoplastic polymer can also be composed of a homopolymer of a vinylidene halide, such as 1,1-dichloroethylene or 1,1-difluoroethylene.
The thermoplastic polymer is preferably a vinyl chloride homopolymer or a poly(butylene terephthalate).
The preferred content of impact additive incorporated in the thermoplastic polymer is between 1 and 30 parts by weight, and preferable between 5 and 10 parts by weight, per 100 parts by weight of the thermoplastic polymer used.
In order to describe the molecular mass of the impact additive, it is possible to define a viscosity in the molten state which varies in the same sense. The said viscosity in the molten state may be situated in a fairly wide range, provided that the impact additive is well dispersed during the operations in which the resin composition, including the said additive, is made use of. As representative magnitude of this viscosity in the molten state, the value of the resisting torque of a Brabender rheometer containing 50 g of impact additive and operating at a temperature of 200xc2x0 C. with a rotational speed of its rotors equal to 40 revolutions per minute may suitably be taken, the torque being determined after holding at 200xc2x0 C. for 20 minutes. Appropriate values of the viscosity in the molten state for the impact additive correspond to values of the abovementioned torque of between 600 and 4000 m.g. In the case of resin compositions for which the thermoplastic polymer is a polymer containing at least 80% by weight of polymerized vinyl chloride, preferred viscosity values in the molten state for the impact additive correspond to values of the said torque ranging from 800 to 3000 m.g. and very particularly from 1000 to 2500 m.g.
Another subject of the invention is a process for producing the said impact additive.
One process comprises the preparation, in a first state, of a crosslinked core composed of a nucleus and of a covering and then, in a second stage, a poly(alkyl methacrylate) shell is grafted onto the said crosslinked core obtained in the first stage.
According to a preferred method, the crosslinked core, composed of a nucleus and of a covering, is prepared and the grafting operation is carried out by using emulsion polymerization techniques. In this case, the following procedure can be used.
In a first stage, an emulsion is prepared which contains, per part by weight of monomers to be polymerized, 1 to 10 parts of water, 0.001 to 0.03 parts of an emulsifying agent, a major portion of the n-alkyl acrylate or of the mixture of alkyl acrylates as defined above to be polymerized in order to form the said core and at least one polyfunctional crosslinking agent. The reaction mixture thus formed is stirred and maintained at a temperature ranging from 55xc2x0 C. to 65xc2x0 C. and preferably at a temperature in the region of 60xc2x0 C. 0.001 to 0.5 parts of a catalyst which generates free radicals is then added and the reaction mixture thus formed is maintained at a temperature of, for example, between ambient temperature and 100xc2x0 C. and with stirring for a period sufficient to obtain a virtually complete conversion of the monomers. The minor portion of n-alkyl acrylate or of the mixture of alkyl acrylates and the grafting agent, as well as, at the same time, 0.001 to 0.005 part of a catalyst which generates free radicals, are then added simultaneously to the phase thus obtained.
This second operation of the first stage, which comprises the production of the covering, is generally carried out at a temperature greater than that used for the preparation of the nucleus. This temperature is not greater than 100xc2x0 C. and preferably between 60xc2x0 C. and 90xc2x0 C.
An alternative form of this first stage comprises the production of the crosslinked core in a single operation by simultaneously introducing the crosslinking agent and the grafting agent (or a compound which plays both the crosslinking role and the grafting role) into the reaction mixture.
In a second stage, the said core is grafted with an alkyl methacrylate. To do this, an appropriate amount of the said methacrylate is added to the reaction mixture resulting from the first stage, in order to obtain a grafted copolymer containing the desired content of grafted chains, as well as, if appropriate, additional amounts of emulsifying agent and of a radical catalyst also within the ranges defined above, and the mixture thus formed is maintained at a temperature within the abovementioned range, with stirring, until virtually complete conversion of the grafting monomers is obtained.
Use may be made, as emulsifying agent, of any one of the known surface-active agents, whether anionic, nonionic or even cationic. In particular, the emulsifying agent may be chosen from anionic emulsifying agents, such as sodium or potassium salts of fatty acids, in particular sodium laurate, sodium stearate, sodium palmitate, sodium oleate, mixed sulphates of sodium or of potassium and of fatty alcohols, in particular sodium lauryl sulphate, sodium or potassium salts of sulphosuccinic esters, sodium or potassium salts of alkylarylsulphonic acids, in particular sodium dodecylbenzenesulphonate, and sodium or potassium salts of fatty monoglyceride monosulphonates, or alternatively from nonionic surfactants, such as the reaction products of ethylene oxide and of alkylphenol or of aliphatic alcohols, alkylphenols. Use may also be made of mixtures of such surface-active agents, if need be.
The catalysts capable of being employed, both in the abovementioned first emulsion polymerization stage and in the abovementioned second emulsion polymerization stage, are compounds which give rise to free radicals under the temperature conditions chosen for the polymerization. These compounds can in particular be peroxide compounds, such as hydrogen peroxide; alkali metal persulphates and in particular sodium or potassium persulphate; ammonium persulphates; percarbonates; peracetates, perborates; peroxides such as benzoyl peroxide or lauroyl peroxide; or hydroperoxides such as cumene hydroperoxide, diisopropylbenzene hydroperoxide, para-menthane hydroperoxide or tert-butyl hydroperoxide.
However, it is preferable to use catalytic systems of redox type formed by the combination of a peroxide compound, for example as mentioned above, with a reducing agent, in particular such as alkali metal sulphite, alkali metal bisulphite, sodium formaldehyde sulphoxylate (NaHSO2, HCHO), ascorbic acid, glucose, and in particular those of the said catalytic systems which are water-soluble, for example potassium persulphate/sodium metabisulphite or alternatively diisopropylbenzene hydroperoxide/sodium formaldehyde sulphoxylate.
It is also possible to add, to the polymerization mixture of one and/or other of the stages, chain-limiting compounds, and in particular mercaptans such as tert-dodecyl mercaptan, isobutyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan or isooctyl mercaptopropionate, for the purpose of controlling the molecular mass of the core and/or of the chains grafted onto the nucleus, or alternatively compounds such as phosphates, for the purpose of controlling the ionic strength of the polymerization mixture.
The reaction mixture obtained on conclusion of the second emulsion polymerization stage, which is composed of an aqueous emulsion of the additive according to the invention, is then treated in order to separate the said additive therefrom. To do this, it is possible, for example, to subject the emulsion, according to the surfactant used, to a coagulating treatment by bringing into contact with a saline solution (CaCl2 or AlCl3) or a solution acidified with concentrated sulphuric acid and then to separate, by filtration, the solid product resulting from the coagulating, the said solid product then being washed and dried to give a graft copolymer as a powder. It is also possible to recover the additive contained in the emulsion by using a spray-drying technique.
The resulting additive exists in the form of a powder, the particle size of which can range from a few microns, for example 0.05 to 5 microns, to 200 to 300 microns, the said particle size depending on the technique used to separate the graft copolymer from the emulsion polymerization mixture.
The composition according to the invention can be prepared by any method which makes it possible to produce a homogeneous mixture containing a thermoplastic polymer, the impact additive according to the invention and optionally other additives. It is possible, for example, to dry-mix the ingredients constituting the resin composition, then to extrude the resulting mixture and to reduce the extrudate to pellets. When the thermoplastic polymer is obtained by emulsion polymerization, it may be convenient to mix the emulsion containing the additive according to the invention with the emulsion of the thermoplastic polymer and to treat the resulting emulsion in order to separate therefrom the solid product which it contains, as described above with respect to the separation of the additive.
The additives other than the impact additive, which may optionally be present in the resin compositions according to the invention are in particular those such as pigments, dyes, plasticizers, antioxidants, heat stabilizers, processing additives or lubricants.
The PVC composition obtained according to the present invention exhibits excellent impact strength at ambient temperature as well as at temperatures as low as xe2x88x9230xc2x0 C. or even xe2x88x9240xc2x0 C.
The composition of the present invention can advantageously be used to produce sections or claddings used in particular in the building industry or alternatively to produce pipes which can be used for conveying water.
The following examples illustrate the invention.